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EP 0 175 026 B1 |
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EUROPEAN PATENT SPECIFICATION |
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Mention of the grant of the patent: |
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28.02.1990 Bulletin 1990/09 |
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Date of filing: 18.09.1984 |
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Marine seismic sensor
Seeseismischer Fühler
Capteur sismique marin
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Designated Contracting States: |
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BE CH DE FR GB IT LI NL SE |
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Date of publication of application: |
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26.03.1986 Bulletin 1986/13 |
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Proprietor: Western Atlas International, Inc. |
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Houston Texas 77042 (US) |
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Inventor: |
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- Savit, Carl H.
Houston
Texas 77252 (US)
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Representative: Godsill, John Kenneth et al |
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Haseltine Lake & Co.,
Imperial House,
15-19 Kingsway London WC2B 6UD London WC2B 6UD (GB) |
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References cited: :
EP-A- 0 027 540 GB-A- 2 083 221 US-A- 4 078 223
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CA-A- 1 124 384 US-A- 3 148 351
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Note: Within nine months from the publication of the mention of the grant of the European
patent, any person may give notice to the European Patent Office of opposition to
the European patent
granted. Notice of opposition shall be filed in a written reasoned statement. It shall
not be deemed to
have been filed until the opposition fee has been paid. (Art. 99(1) European Patent
Convention).
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[0001] This invention relates to the use of pressure sensors to determine the direction
of propagation of seismic pressure waves in a body of water.
[0002] In seismic exploration at sea, a plurality of pressure sensors are encased in a long
tubular plastic streamer which may extend for 1.6 to 3.2 km. A ship tows the streamer
through the water at a desired depth. The earth layers beneath the sea are insonified
by suitable means. The sonic waves are reflected from the earth layers below, to return
to the surface of the water in the form of pressure waves. The pressure waves are
detected by the pressure sensors and are converted to electrical signals. The electrical
signals are transmitted to the towing ship via transmission lines that are contained
within the streamer.
[0003] The reflected sound waves not only return directly to the pressure sensors where
they are first detected, but those same reflected sound waves are reflected a second
time from the water surface and back to the pressure sensors. The surface-reflected
sound waves of course, are delayed by an amount of time proportional to twice the
depth of the pressure sensors and appear as secondary or "ghost" signals. Because
the direct and surface-reflected sound waves arrive close together in time-a few milliseconds-they
tend to interfere with one another. It is desirable therefore to determine the direction
of propagation of the sound waves so that the upward- and downward-propagating waves
may be more readily sorted out during data processing.
[0004] It is possible to position two individual sensors in a fixed vertical array. It would
of course then be easy to identify the direction of propagation of the sonic waves
from the measured difference in time that a particular wavelet arrives at the respective
sensors that make up the vertical array. See for example, U.S. Patent 3,952,281. That
method however requires two separate hydrophone cables. Since such cables cost about
a half-million dollars each, that course of action would be decidedly uneconomical.
[0005] Assuming that sufficiently compact sensors could be obtained, it would be possible
to mount a substantially vertical array of sensors inside the same streamer, a few
centimeters or inches apart. But a seismic streamer cable twists and turns as it is
towed through the water. If a substantially vertical sensor array were to be mounted
inside the streamer, there would no way to determine which one of the sensors in the
array is "up", assuming conventional detectors are used. It is also important to be
able to identify unwanted waves travelling horizontally from scatterers within or
near the bottom of the water layer.
[0006] As is well known, a water-pressure gradient exists between two points spaced vertically
apart in a body of water. If then, there were some way that the hydrostatic pressure
gradient between two vertically-disposed detectors could be measured, the uppermost
detector of an array could be identified.
[0007] A pressure gradient transducer for a directional hydrophone system is disclosed in
US-A-3148351. The transducer is for indicating its orientation relative to magnetic
north.
[0008] The present invention relates to a method for marine seismic exploration comprising
pulling an elongate member through a body of water in a substantially horizontal configuration,
the member having a plurality of sensor arrays disposed within and along it for producing
data indicative of seismicwaves, each array comprising at least three sensors displaced
radially in said elongate member in different directions, with the sensors in each
array being substantially in a plane orthogonal to the elongate member, and the method
further comprising providing the data in the form of electrical signals.
[0009] U.S. Specification No. 4078223 describes such a method, with the possible exception
that it is not clear whether the sensors are disposed in said orthogonal plane. Moreover,
this method does not relate to what we call marine exploration, i.e. where an elongate
member is towed through a body of water at a finite depth above the bed of that body
of water. Instead, it is a method for use on 'land' (which may include the bed of
a shallow lake orthe like) using geophones which can only detect signals propagating
substantially along their axes of sensitivity and which are directionally insensitive.
The type of geophone disclosed was operative only with its axis within a few degrees
of the vertical, so that the method renders only one sensor operational at a time.
[0010] According to one aspect of the present invention, the method is characterised in
that it is carried out with the elongate member towed through the body of water at
a preselected finite depth above the bed of that body of water, the sensors being
of a type to respond to the magnitude and direction of pressure thereon so as to provide
data having AC and DC components indicative of the seismic waves and of ambient hydrostatic
pressures respectively, the method further comprising separating the AC and DC components
of the data into electrical signals and processing the electrical signals to resolve
the AC and DC components to determine the direction of propagation of said seismic
waves with respect to three-dimensional space.
[0011] US Specification No. 4078223 describes apparatus for carrying out its method, the
apparatus including an elongate member containing at least one sensor array, said
array including at least three radially disposed sensors to produce data signals relating
to detected seismic waves, the sensors of the or each array being disposed in different
directions substantially in a plane orthogonal relative to the elongate member, means
for transmitting an actuating signal to said sensors and means for transmitting data
signals from the sensors to a recording system.
[0012] According to a second aspect of the present invention, such an apparatus is characterised
in that, for use with the elongate member towed through a body of water in a substantially
horizontal configuration at a preselected finite depth above the bed of that body
of water, the sensors are of a type to respond to the magnitude and direction of pressure
thereon so as to be operable to produce data signals related to differential hydrostatic
pressure and sensed pressure transients due to seismic waves, the apparatus further
comprising means for separating the data signals into electrical DC components representative
of the hydrostatic pressure and electrical AC components representative of the transients
due to seismic waves, and means for determining from the components the direction
of propagation of the detected seismic waves with respect to three-dimensional space.
[0013] Conventional marine detectors or hydrophones use piezo-electric ceramic wafers as
the active element. The wafers are generally mounted to operate in the bender mode.
Transient pressure changes due to acoustic waves flex the wafers to generate an AC
charge current. The wafers are also sensitive to hydrostatic pressure, but the DC
charge due to hydrostatic pressure leaks off rapidly through associated circuitry.
[0014] A preferred form of the invention uses sensors of fiber-optic type. Canadian Patent
No. 1124384 uses such sensors. It discloses a sensor array, the sensor array comprising
two pressure sensors of fiber-optic type displaced radially from an axis of the array
and disposed in substantially the same plane orthogonal to said axis, means for transmitting
an actuating signal to the sensors and means for transmitting data signals from said
sensors. Such an array as such is not suitable for implementing the present invention.
[0015] Thus, according to a further aspect of the invention defined in claim 13, there are
at least three of said sensors of fiber-optic type arranged with their axes extending
substantially along different radii of the array such that said sensors will produce
data signals comprising DC components representative of the magnitude of hydrostatic
pressure and AC components representative of transients due to seismic waves.
[0016] Reference has been made above in describing the invention to AC and DC signals. The
term DC as used herein applies to the component of the signals generated by the hydrostatic
pressure. With fiber optic type sensors, the different hydrostatic pressures exerted
on two sensors can be distinguished by measuring the phase shift between the sensors'
interference fringes. This results in a signal of constant polarity referred to as
the "DC component". However, the marine environment is not static. Waves and swells
are present which continually change the absolute hydrostatic pressure detected, but
nevertheless this is regarded as DC herein. In contrast, the AC component relates
to the transient pressure pulses due to seismic waves.
[0017] A preferred embodiment of the present invention seeks to provide a plurality of arrays
of pressure sensors in an inexpensive streamer that is capable of detecting AC transient
pressure signals due to seismic waves and to identify their direction of arrival in
three dimensional space with reference to the vertical whose direction is sensed by
measuring the DC bias due to the vertical hydrostatic pressure gradient.
[0018] In accordance with such an embodiment, a plurality of optical-fiber sensor arrays
are mounted interiorly of a seismic streamer at a like plurality of sensor stations
distributed at intervals along the stramer. Each sensor array consists of a set of
at least three and preferably four coils of monomodal optical fiber that act as pressure
sensors. If four coils are used, the four sensor-coils may be mounted ninety degrees
apart around the inner surface of the streamer skin. A laser or LED launches a coherent
beam of monochromatic light into each set of sensor coils via an input transmission
line. Transient and static pressures at the sensor coils modulate the light beam.
The modulated output light beam from each sensor coil of a set is delivered by at
least one optical fiber to a multiple-input photo detector where the beam from each
individual sensor coil is separately combined with a reference beam. The photo-detector
converts the resulting optical beat signals to AC electrical signals representative
of the polarity and amplitude of transient seismic signals impinging upon the sensor
coils.
[0019] Preferably, separate modulated output light beams are combined with each other at
a photo-detector which converts the phase difference between the light beams to a
DC electrical signal having a magnitude representative of the DC bias due to the hydrostatic
pressure gradient between the sensor coils. The AC seismic signals and the DC bias
signals are transmitted to a data processor where the direction of propagation of
incoming seismic waves may be resolved.
[0020] The laser, photo-detectors, data processor and other optical and electronic circuitry
can be mounted aboard a towing ship. The input and modulated output light beams are
transmitted to the sensor coils through optical-fiber bundles.
[0021] Preferably, each set of sensor-coils is provided with a separate laser or LED, photo
detectors, and a beam splitter to provide a reference beam all mounted together in
a single module at the sensor stations. The modulated light beams are resolved as
to the AC and DC signal components which are converted to electrical signals. The
electrical signals are transmitted to the data processor by wire line.
[0022] For a better understanding of this invention, reference may be made to the appended
detailed description and the drawings wherein:
Figure 1 shows a boat towing through the water a streamer containing a plurality of
optical-fiber sensor coils at corresponding sensor stations;
Figure 2 is a longitudinal cross section of the streamer at a typical sensor station;
Figure 3 is a cross section of the streamer along line 3-3; and
Figure 4 illustrates schematically, the optical circuitry.
[0023] Referring now to Figure 1, there is shown a ship 10 towing a seismic streamer 12
through a body of water 14. Streamer 12 is towed by an armored lead-in 16 which includes
stress members, armoring and it may include one or more optical fiber bundles. When-not
in use, lead-in 16 and streamer 12 are stored on a reel 18 at the stern of boat 10.
Streamer 12 contains several sets 20 of optical-fiber sensor coils, one set per sensor
station. As will be seen later, each set 20, includes three but preferably four such
sensor coils. An optical equipment package 22 such as a laser, photo detectors, optical
couplers and data processing equipment is mounted aboard ship 10. Equipment package
22 will be described at length later. A tail buoy or drogue 24 marks the end of the
streamer 12. One known system, which however employs only one sensor per sensor station
is shown in U.S. Patent 4,115,753.
[0024] Streamer 12 consists essentially of a long tubular plastic skin made of polyvinyl
chloride, polyurethane or the like, about 76 mm in diameter, closed at both ends.
A complete streamer may be several kilometers long but, for convenience in handling,
it may be divided into a number of detachable sections. The streamer is filled with
a substantially incompressible fluid transparent to seismic waves for coupling external
pressures in the internally-mounted sensors. A stress member 28, usually a stainless
steel cable, is threaded through the entire streamer to prevent rupture due to towing
stress.
[0025] Referring to Figure 2 which is a longitudinal cross section of a portion of the cable
at a sensor station, and Figure 3 which is a cross section at 3-3 of Figure 2, a sensor
unit 20 consists of a set of at least three and preferably four optical-fiber sensor
coils 30, 32, 34, 36 having an elongate configuration that are mounted inside skin
26 of streamer 12 parallel to the longitudinal axis. For sake of example, let it be
assumed that there are four such coils. There are thus two pairs of sensor coils such
as 30 and 32, 34 and 36. The members of each pair are mounted diametrically opposite
to one another at 90° intervals, parallel to and as far away from the longitudinal
axis of the streamer as practicable. Preferably the sensor coils are held in place
by a plastic spider such as 38. Since the longitudinal axis of the streamer, when
under low, is substantially horizontal, the set of sensor coils forms a two-dimensional
array having a vertical extent comparable to the inner diameter of the tube 26.
[0026] The sensor coils are fashioned from many turns of a monomodal glass fiber having
a low light loss per unit of length. The dimensions of the coil and the number of
turns depend upon the total length of optical fiber required.
[0027] It is well known that when an optical fiber is subjected to a compression, the index
of refraction and/or the elongation changes. The phase shift between a light beam
transmitted through a reference fiber and a beam transmitted through an active fiber
subjected to compression is a function of the fiber length and the incremental change
in the index of refraction and/or elongation or both. See for example, U.S. Patent
4,320,475. For a practical pressure sensor, a fiber length of about 100 meters is
required for the active fiber. For an elongated fiber coil loop about two meters long
and two or three centimeters wide, about 25 turns would be necessary. It is necessary
for the sensor coils to be mounted so that flexing or movement of the streamer skin
will not distort the shape of the coils. Such distortion would of course introduce
spurious signals to the system.
[0028] Two optical-fiber bundles 40 and 42 are threaded through the streamer and the respective
spiders that support the sensor coils at each sensor station. Bundle 40 is the outbound
transmission link through which is launched an input light beam from a transmitting
laser (not shown in Figure 2), to each sensor coil. Bundle 40 may be a single fiber
with provision for coupling its transmitted light to each sensor coil or it may consist
of a bundle of single fibers, one fiber being assigned to each sensor coil. In effect
the coils have an essentially common light-beam input. For example, coil 36 has an
input fiber lead 35 and an output fiber lead 37. The other coils have similar input
and output leads. Because of the small size and light weight of the fibers, several
hundred fibers can be packaged into a single bundle without becoming unduly bulky.
[0029] Fiber bundle 42 is the return transmission link for the sensor-coil output light
beams. There is one output fiber for every sensor-coil. Therefore, four output fibers
are necessary to service each sensor station. The free end of fiber bundle 42 that
exits the streamer and lead-in at the ship, is coupled to optical processing circuitry
now to be described.
[0030] The preferred method of operation of this invention may be gleaned from Figure 4
which schematically illustrates the optical processing circuitry. In Figure 4, all
components to the left of dashed line 44 may be mounted on ship 10 as part of the
processing package 22. Components to the right of dashed line 44 are made a part of
streamer 12.
[0031] A laser or LED 46, operating preferably in the near infra-red portion of.the spectrum
launches a coherent light beam 47 into an optical coupler 48 that couples the light
beam into the fiber or fibers that make up fiber bundle 40. The optical coupler 48
acts as an essentially common input to the fiber bundle. The light is transmitted
to the optical-fiber sensor coils where the light beams are modulated by transient
seismic pressure waves and the ambient hydrostatic pressure. The modulated light beams
return from the sensor coils, through fiber bundle 42, to processing unit 22. In Figure
4, only one typical sensor station is shown for simplicity, but it should be understood
that fiber bundles 40 and 42 may be extended to service additional sensor stations.
[0032] In optical equipment package 22, a beam splitter 50 directs a part 53 of the laser
beam 47 into a suitable optical delay module 52 whose output becomes a reference beam
54. Optical delay module 52 retards beam 53 to match the length of the optical path
between beam splitter 50 and the sensor coils 30, 32, 34, 36 of any given sensor station.
A different delay module is associated with each of the plurality of sensor stations
to compensate for the differing optical path lengths.
[0033] The modulated light beams return from sensor coils 30, 32, 34, 36 through corresponding
optical fibers 30', 32', 34', 36'. The beams are individually combined with reference
beam 54 by suitable photo-detectors, of any desired type, in multiple-input combiner
module 56. The resulting beat frequency is converted to an AC electrical wave train
representative of the transient pressure variations due to seismic waves. The electrical
signals from the four sensors may be multiplexed into data processor 58 over line
60.
[0034] The DC bias, due to a water-pressure gradient, between the light beams in a first
pair of diametrically opposite sensor coils such as 30 and 32 is measured by combining
the two output light beams in a photo-detector 62. The phase shift between the two
beams is converted to a DC electric bias signal having sign and magnitude that is
delivered to data processor 58 over line 64. Similarly the DC bias between the light
outputs of the second pair of coils, 34 and 36, is measured by photo detector 66.
The resulting electrical output is transmitted to data processor 58 over line 68.
From the magnitude of the two bias signals, the physical orientation of the sensor
coils, relative to a vertical plane, can be resolved by well known mathematical algorithms.
In data processor 58, since we know now of the physical orientation of the sensor
coils in the vertical plane, the directions of propagation of the respective seismic
pressure waves can be resolved by measuring the arrival-time differences of a seismic
wavelet at the respective sensor coils of the array.
[0035] In the above discussion, I have disclosed a means for resolving the magnitude and
direction, within a vertical plane perpendicular to the axis of the cable, of seismic
waves propagating through water. The direction of propagation in three-dimensional
space can of course be determined by measuring the time difference between the arrival
times of the same seismic wavelet at two or more selected consecutive sensor stations
along the cable by means well known to the art. The longitudinal time differences
may be combined with the vertical time differences by simple vector addition to resolve
the direction of propagation in three axes.
[0036] I have described my invention in terms of a specific configuration. However, those
skilled in the art may consider other equally effective arrangements without departing
from the scope of the appended claims. For example, each of the individual sensor
arrays could be provided with its own laser, beam splitter, photo-detectors etc.,
all of which could be included in individual modules mounted in the streamer at each
sensor station. The electrical analogs of the measured phase shifts of the modulated
and reference light beams would be transmitted to data processor 58, aboard ship 10,
by wire line.
1. A method for marine seismic exploration comprising pulling an elongate member through
a body of water in a substantially horizontal configuration, the member having a plurality
of sensor arrays disposed within and along it for producing data indicative of seismic
waves, each array comprising at least three sensors displaced radially in said elongate
member in different directions with the sensors in each array being substantially
in a plane orthogonal to the elongate member, and the method further comprising providing
the data in the form of electrical signals, characterised in that the method is carried
out with the elongate member towed through the body of water at a preselected finite
depth above the bed of that body of water, the sensors being of a type to respond
to the magnitude and direction of pressure thereon so as to provide data having AC
and DC components indicative of the seismic waves and of ambient hydrostatic pressures
respectively, the method further comprising separating the AC and DC components of
the data into electrical signals and processing the electric signals to resolve the
AC and DC components to determine the direction of propagation of said seismic waves
with respect to three-dimensional space.
2. The method of claim 1, wherein in each array pressure sensors substantially uniformly
distributed around the longitudinal axis of said elongate member are used.
3. The method of claim 1 or 2, wherein in each array optical-fiber pressure sensors
(30, 32, 34) for receiving and modulating a coherent monochromatic light beam are
used, thereby providing a plurality of output light beams that are modulated in response
to transient pressure variations due to seismic waves and in response to hydrostatic
pressure.
4. The method of claim 3, and comprising deriving AC signal components representative
of said transient pressure variations by separately combining said modulated light
beams with a reference light beam, and for each sensor array, combining the separate
modulated light beams with each other to derive DC signal components representative
of the magnitude of the hydrostatic pressure differential between said sensors.
5. The method of any one of claims 1 to 4, and comprising measuring the horizontal
and vertical arrival-time differences of the transient pressure variations at selected
horizontally and radially disposed sensors making up the respective sensor arrays,
and combining said AC components, said DC components and said arrival times for resolving
the direction of propagation, in three-dimensional space, of the detected seismic
waves.
6. An apparatus for carrying out the method of any one of the preceding claims and
including an elongate member (12) containing at least one sensor array (20), said
array (20) including at least three radially disposed sensors (30, 32, 34) to produce
data signals relating to detected seismic waves, the sensors (30, 32, 34, 36) of the
or each array (20) being disposed in different direction substantially in a plane
orthogonal relative to the elongate member (12) means (40) for transmitting an actuating
signal to said sensors (30, 32, 34) and means (42) for transmitting data signals from
the sensors to a recording system (22), characterised in that for use with the elongate
member towed through a body of water in a substantially horizontal configuration at
a preselected finite depth above the bed of that body of water, the sensors (30, 32,
34, 36) are of a type to respond to the magnitude and direction of pressure thereon
so as to be operable to produce data signals related to differential hydrostatic pressure
and sensed pressure transients due to seismic waves, the apparatus further comprising
means (56, 62) for separating the data signals into electrical DC components representative
of the hydrostatic pressure and electrical AC components representative of the transients
due to seismic waves, and means (58) for determining from the components the direction
of propagation of the detected seismic waves with respect to three-dimensional space.
7. The apparatus of claim 6, wherein a plurality of sensor arrays in spaced apart
along the longitudinal axis of said elongate member (12) each said array (20) including
four optical fiber sensors (30, 32, 34, 36), said sensors being uniformly distributed
about the longitudinal axis of said elongate member (12) in a plane substantially
perpendicular to the longitudinal axis of said member, said actuating-signal transmitting
means (40) being an optical fiber and said data- signal transmitting means (42) including
at least one optical fiber.
8. The apparatus of claim 7, and comprising a laser (46) for launching a coherent
monochromatic beam of radiation into the transmission means (40) for modulation by
the sensors of the respective arrays (20) in response to pressure variations due to
seismic waves and to differential hydrostatic pressure, a detector means (56) of the
recording system for receiving separately the modulated light beams from the respective
sensors (30,32,34,36) of each array (20) over the data transmitting means (42) and
for combining said modulated light beams with a reference light beam (54) to determine
the AC signal components of said modulated light beam that are due to pressure transients
caused by seismic waves.
9. The apparatus according to claim 7 or 8 and comprising photo detectors (62, 66)
for combining respectively the modulated light beams from a pair of the sensors (30
and 32) and from another pair of the sensors (34 and 36) to determine the DC components
of said modulated light beams that are representative of differential hydrostatic
pressure at the respective sensors.
10. The apparatus according to any one of claims 7 to 9, wherein all of the sensors
of all of the respective arrays (20) have a common source of coherent radiation, each
sensor being arranged to provide a separate modulated output beam, and said data transmission
means (42) comprising a bundle of separate optical fibers.
11. The apparatus according to any one of claims 7 to 9, wherein all of the sensors
within a given array (20) have a common radiation source but each separate array has
a separate radiation source, all of the sensors of all of the arrays being arranged
to provide separate modulated output beams.
12. The apparatus according to anyone of claims 6 to 11, wherein said elongate member
(12) is a closed container having a volume of fluid contained therewithin for coupling
the internally-mounted sensors with external pressure variations.
13. A sensor array (20) for use in an apparatus according to claim 6, the sensor array
(20) comprising pressure sensors of fiber-optic type displaced radially from an axis
of the array (20) and disposed in substantially the same plane orthogonal to said
axis, means (40) for transmitting an actuating signal to the sensors and means (42)
for transmitting data signals from said sensors, characterised in that there are at
least three of said sensors (30, 32, 34, 31 ) offiber-optictype arranged with their
axes extending substantially along different radii of the array such that said sensors
will produce data signals comprising DC components representative of the magnitude
of hydrostatic pressure and AC components representative of transients due to seismic
waves.
1. Verfahren zur seismischen Exploration auf See, mit den Verfahrensschritten:
Schleppen eines im wesentlichen horizontal auszurichtenden langen Körpers durch eine
Wassermasse, welcher Körper eine Vielzahl von Sensorreihen aufweist, die innerhalb
und längs des Körpers angeordnet sind zur Erzeugung von seismischen Wellen entsprechenden
Daten, und bei dem jede Reihe mindestens drei innerhalb des Körpers in verschiedene
radiale Richtungen ausgerichtete Sensoren enthält und jede Reihe in einer zur Körperlängsachse
im wesentlichen senkrechten Ebene angeordnet ist und
Bereitstellen der Daten in Form von elektrischen Signalen, dadurch gekennzeichnet,
daß
das Verfahren mittels des in einer bestimmten Tiefe oberhalb des Wasserbodens durch
die Wassermasse geschleppten langen Körpers durchgeführt wird, die Sensoren auf die
Stärke und die Richtung des auf sie wirkenden Druckes ansprechen und Daten bereitstellen,
welche die seismischen Wellen bzw, den hydrostatischen Umgebungsdruck anzeigende Wechselstrom-(AC-)
und Gleichstrom-(DC-) Komponenten enthalten,
bei dem Verfahren des weiteren die AC- und DC-Komponenten der Daten in elektrische
Signale separiert werden und daß
die elektrischen Signale zum Analysieren der AC- und DC-Komponenten verarbeitet werden,
um die Ausbreitungsrichtung der seismischen Wellen im dreidimensionalen Raum zu bestimmen.
2. Verfahren nach Anspruch 1, bei dem jeder Reihe Drücksensoren verwendet werden,
die im wesentlichen gleichmäßig um die Längsachse des langen Körpers angeordnet sind.
3. Verfahren nach Anspruch 1 oder 2, bei dem in jeder Reihe Optikfaser-Drucksensoren
(30, 32, 34) verwendet werden, die einen kohärenten monochromatischen Lichtstrahl
empfangen und mnodulieren zur Bereitstellung von einer Vielzahl von Ausgangs-Lichtstrahlen,
die in Abhängigkeit der aufgrund von Seismikwellen entstandenen transienten Druckänderungen
und in Abhängigkeit des hydrostatischen Druckes moduliert sind.
4. Verfahren nach Anspruch 3, mit den Verfahrensschritten:
Gewinnung der die transierten Druckänderungen darstellenden AC-Komponenten durch separate
Vereinigung der modulierten Lichtstrahlen mit einem Bezugs-Lichtstrahl und
Vereinigen der separaten modulierten Lichtstrahlen miteinander für jede Sensorreihe
zur Gewinnung der die Stärke der hydrostatischen Druckdifferenz zwischen den Sensoren
repräsentierenden DC-Komponenten.
5. Verfahren nach einem der Ansprüche 1 bis 4, mit den Verfahrensschritten:
Messen der horizontalen und vertikalen Ankunftszeitunterschiede der transienten Drukkänderungen
an ausgewählten horizontal und radial angeordneten Sensoren, die die entsprechenden
Sensorreihen bilden, und
Kombinieren der AC-Komponenten, der DC-Komponenten und der Ankunftszeiten zum Bestimmmen
der Ausbreitungsrichtung der erfaßten seismischen Wellen im dreidimensionalen Raum.
6. Vorrichtung zur Durchführung des Verfahrens nach einem der vorstehenden Ansprüche
mit:
einem langen Körper (12) mit mindestens einer Sensorreihe (20), die mindestens drei
radial angeordnete Sensoren (30, 32, 34) enthält zur Erzeugung von seismischen Wellen
entsprechenden Datensignalen, wobei die Sensoren (30, 32, 34, 36) einer jeden Reihe
(20) innerhalb einer bezüglich des langen Körpers (12) orthogonalen Ebene in unterschiedlichen
Richtungen angeordnet sind,
Mittels (40) zur Übertragung eines Aktivierungssignals zu den Sensoren (30, 32, 34)
und mit:
Mitteln (42) zum Übertragen der Datensignale von den Sensoren zu einem Aufzeichungssystem
(22), dadurch gekennzeichnet, daß
für den in einer bestimmten Tiefe oberhalb des Wasserbodens durch die Wassermasse
zu schleppenden langen körpers Sensoren (30, 32, 34, 36) verwendet werden, die auf
die Stärke und die Richtung des auf sie wirkenden Drucks ansprechen zur Erzeugung
der Datensignale aufgrund des hydrostatischen Druckdifferenz, die durch die seismischen
Wellen ausgelöst werden, und der erfaßten Drucktransienten,
die Vorrichtung des weiteren,
Einrichtungen (56, 62) zum Trennen der Datensignale in den hydrostatischen Druck repräsentierende
elektrische DC-Komponenten und in die Drucktransienten, die durch die seismischen
Wellen entstehen, darstellende elektrische AC-Komponenten und
Mittel (58) enthält zur Bestimmung der Ausbreitungsrichtung der erfaßten seismischen
Wellen im dreidimensionalen Raum aus den Komponenten.
7. Vorrichtung nach Anspruch 6, bei der eine Vielzahl von Sensorreihen entlang der
Längsachse des langen Körpers (12) voneinander beabstandet angeordnet sind, bei der
zu jeder Reihe (20) vier faseroptische Sensoren (30, 32, 34, 36) gehören, die in einer
im wesentlichen zur Längsachse des Körpers senkrechten Ebene gleichmäßig um die Längsachse
des langen Körpers (12) verteilt sind, und bei der das Mittel (40) zur Übertragung
des Aktivierungssignals eine optische Faser ist und die Einrichtung (42) zur Übertragung
des Datensignals mindestens eine optische Faser enthält.
8. Vorrichtung nach Anspruch 7, mit
einem Laser (46), der einen kohärenten monochromatischen Lichtstrahl in das Mittel
(40) zur Übertragung einstrahlt, welcher durch die Sensoren der entsprechenden Reihe
(20) in Abhängigkeit von der durch die seismischen Wellen bewirkten Druckänderungen
und in Abhängigkeit vom hydrostatischen Druckdifferenzwert moduliert wird,
einem Detektor (56) im Aufzeichnungssystem, der über die Einrichtung (42) zur Datenübertragung
die modulierten Lichtstrahlen von den entsprechenden Sensoren (30, 32, 34, 36) jeder
Reihe (20) aufnimmt und diese modulierten Lichtstrahlen mit einem Referenzlichstrahl
(54) vereinigt zur Bestimmung derAC-Komponenten des modulierten Lichtstrahls, die
abhängig sind von den durch die seismischen Wellen bewirkten Drucktransienten.
9. Vorrichtung nach Anspruch 7 oder 8, mit
Photodetektoren (62, 66), welche die entsprechenden modulierten Lichtstrahlen eines
Sensorpaars (30 und 32) und eines weiteren Sensorpaares (34 und 36) vereinigen zur
Bestimmung der DC-Komponente der modulierten Lichtstrahlen, welche die hydrostatische
Druckdifferenz an den entsprechenden Sensoren representieren.
10. Vorrichtung nach einem der Ansprüche 7 bis 9, bei der allen Sensoren aller Reihen
(20) eine gemeinsame Quelle für kohärente Strahlung zugeordnet ist, bei der jeder
Sensor so angeordnet ist, daß er einen separat modulierten Strahl ausgibt und bei
der die Einrichtung (42) zur Datenübertragung ein Bündel einzelner optische Fasern
aufweist.
11. Vorrichtung nach einem der Ansprüche 7 bis 9, bei der allen Sensoren innerhalb
einer bestimmten Reihe (20) eine gemeinsame Strahlungsquelle zugeordnet ist, aber
jede einzelne Reihe eine getrennte Strahlungsquelle enthält und bei der sämtliche
Sensoren aller Reihen derart angeordnet sind, daß sie getrennte modulierte Strahlen
ausgeben.
12. Vorrichtung nach einem der Ansprüche 6 bis 11, bei der der lange Körper (12) ein
geschlossener Behälter ist und ein Fluid enthält zur Übertragung der äußeren Druckänderungen
auf die in seinem Inneren befestigten Sensoren.
13. Sensorreihenanordnung (20) zur Verwendung in einer Vorrichtung nach Anspruch 6
mit
faseroptischen Drucksensoren, die von einer Achse der Reihe (20) radial beabstandet
und im wesentlichen in der gleichen zu dieser Achse orthogonalen Ebene angeordnet
sind,
Einrichtungen (40) zur Übertragung eines Aktivierungssignals zu den Sensoren und mit
Einrichtungen (42) zur Übertragung von Datensignalen von den Sensoren, dadurch gekennzeichnet,
daß
mindestens drei der faseroptischen Sensoren (30, 32, 34,31) mit ihren Achsen im wesentlichen
entlang unterschiedlicher Radien der Reihe so angeordnet sind, daß Datensignale erzeugt
werden, welche die Stärke des hydrostatischen Drucks repräsentierende DC-Komponente
und AC-Komponenten enthalten, welche den von den seismischen Wellen bewirkten Drucktransienten
entsprechen.
1. Un procédé d'exploration sismique marine comprenant le remorquage dans une configuration
sensiblement horizontale d'un élément allongé dans une masse d'eau, l'élément comprenant
une pluralité de réseaux de capteurs disposés à l'intérieur et le long de celui-ci
pour générer des données d'indication d'ondes sismiques, chaque réseau comprenant
au moins trois capteurs décalés radialement dans ledit élément allongé dans des directions
différentes, les capteurs de chaque réseau étant sensiblement dans un plan orthogonal
à l'élément allongé, et le procédé comprenant en outre l'envoi de données sous forme
de signaux électriques, caractérisé en ce que le procédé est exécuté au moyen de l'élément
allongé remorqué dans la masse d'eau à une profondeur finie présélectionnée au dessus
du fond de cette masse d'eau, les capteurs étant d'un type sensible à l'amplitude
et à la direction de la pression qui leur est appliquée, de manière à générer des
données dont les composantes à courant alternatif et à courant continu sont une indication
des ondes sismiques et des pressions hydrostatiques ambiantes, respectivement, le
procédé comprenant en outre la séparation des composantes à courant alternatif et
à courant continu des données en signaux électriques et le traitement des signaux
électriques pour calculer les composantes à courant alternatif et a courant continu
pour déterminer la direction de propagation desdites ondes sismiques par rapport à
l'espace tri-dimensionnel.
2. Procédé selon la revendication 1, dans lequel on utilise dans chaque réseau des
capteurs de pression distribués de façon sensiblement uniforme autour de l'axe longitudinal
dudit élément allongé.
3. Procédé selon la revendication 1 ou 2, dans lequel on utilise dans chaque réseau
des capteurs de pression à fibres optiques (30, 32, 34) pour recevoir et moduler un
faisceau lumineux monochromatique cohérent, ce qui donne ainsi une pluralité de faisceaux
lumineux de sortie en réponse modulée aux variations de pression transitoire dues
aux ondes sismiques et en réponse à la préssion hydrostatique.
4. Procédé selon la revendication 3, et comprenant la dérivation des composantes de
signaux à courant alternatif représentatives desdites variations de pression transitoire
en combinant séparément lesdits faisceaux lumineux modulés à un faisceau lumineux
de référence, et pour chaque réseau de capteurs, la combinaison des faisceaux lumineux
modulés entre eux pour dériver les composantes des signaux à courant continu représentant
l'amplitude de la pression hydrostatique différentielle entre lesdits capteurs.
5. Procédé selon l'une quelconque des revendications 1 à 4 et comprenant la mesure
des différences des temps d'arrivée horizontaux et verticaux des variations de pression
transitoire à des capteurs sélectionnés, disposés horizontalement et radialement,
constituant les réseaux de capteurs respectifs, et la combinaison desdites composantes
à courant alternatif, desdites composantes à courant continu et desdits temps d'arrivée
pour calculer la direction de propagation, dans l'espace tri-dimensionnel, des ondes
sismiques détectées.
6. Appareil pour exécuter le procédé de l'une quelconque des précédentes revendications
et comprenant un élément allongé (12) contenant un réseau de détecteurs (20) au moins,
chaque réseau (20) comprenant au moins trois capteurs disposés radialement (30, 32,
34) pour générer des signaux de données relatifs aux ondes sismiques détectées, les
capteurs (30, 32, 34, 36) du ou de chaque réseau (20) étant disposés dans différentes
directions, sensiblement dans un plan orthogonal par rapport à l'élément allongé (12),
des moyens (40) pour transmettre un signal de commande auxdits capteurs (30, 32, 34)
et des moyens (42) pour transmettre des signaux de données des capteurs à un système
d'enregistrement (22), caractérisé en ce que pour pouvoir être utilisés avec l'élément
allongé remorqué dans une configuration sensiblement horizontale dans une masse d'eau,
à une profondeur finie présélectionnée au dessus du fond de cette masse d'eau, les
capteurs (30, 32, 34, 36) sont d'un type sensible à l'amplitude et à la direction
de la pression qui leur est appliquée, de manière à pouvoir être actionnés pour produire
des signaux de données en fonction de la pression hydrostatique différentielle et
des transitoires de pression détectées dues aux ondes sismiques, l'appareil comprenant
en outre des moyens (56, 62) pour séparer les signaux de données en composantes à
courant continue électrique représentatives de la pression hydrostatique et en composantes
à courant alternatif électrique représentatives des transitoires dues aux ondes sismiques,
et des moyens (58) pour déterminer, à partir des composantes, la direction de propagation
des ondes sismiques détectées par rapport à l'espace tri-dimensionnel.
7. Appareil selon la revendication 6, dans lequel une pluralité de résaux de détecteurs
sont espacés le long de l'axe longitudinal dudit élément allongé (12), chacun desdits
réseaux (20) comprenant quatre capteurs à fibres optiques (30, 32, 34, 36), lesdits
capteurs étant distribués uniformément autour de l'axe longitudinal dudit élément
allongé (12) dans un plan sensiblement perpendiculaire à l'axe longitudinal dudit
élément, lesdits moyens de transmission des signaux de commande (40) étant une fibre
optique et lesdits moyens de transmission des signaux de données (42) comprenant une
fibre optique au moins.
8. Appareil selon la revendication 7, et comprenant un laser (46) pour lancer un faisceau
monochromatique cohérent de radiations dans les moyens de transmission (40) en vue
d'être modulé par les capteurs des réseaux respectifs (20) en résponse aux variations
de pression dues aux ondes sismiques et à la pression hydrostatique différentielle,
des moyens de détection (56) du système d'enrégistrement pour recevoir séparément
les faisceaux lumineux modulés des capteurs respectifs (30, 32, 34, 36) de chaque
réseau (20) par les moyens de transmission des données (42) et pour combiner lesdits
faisceaux lumineux modulés à un faisceau lumineux de référence (54) pour déterminer
les composantes des signaux à courant alternatif dudit faisceau lumineux modulé dues
aux transitories de pression produites par les ondes sismiques.
9. Appareil selon la revendication 7 ou 8, comprenant des photodétecteurs (62, 66)
pour combiner respectivement les faisceaux lumineux modulés d'une paire de capteurs
(30 et 32) et d'une autre paire de capteurs (34 et 36) afin de déterminer les composantes
à courant continu desdits faisceaux lumineux modulés représentatives de la pression
hydrostatique différentielle aux capteurs respectifs.
10. Appareil selon l'une quelconque des revendications 7 à 9, dans lequel tous les
capteurs de tous les réseaux respectifs (20) ont une source de rayonnement cohérent
commun, chaque capteur étant agencé pour produire un faisceau de sortie modulé séparé,
et lesdits moyens de transmission des données (42) comprenant un faisceau de fibres
optiques séparées.
11. Appareil selon l'une quelconque des revendications 7 à 9, dans lequel tous les
capteurs d'un réseau donné (20) ont une source de radiation commune, mais chaque jeu
séparé a une source de rayonnement séparée, tous les capteurs de tous les réseaux
étant agencés pour produire des faisceaux de sortie modulée séparés.
12. Appareil selon l'une quelconque des revendications 6 à 11, dans lequel ledit élément
allongé (12) est un conteneur fermé contenant à l'intérieur un volume de fluide pour
le couplage des capteurs montés intérieurement avec les variations de pression extérieure.
13. Réseau de capteurs (20) prévu pour être utilisé dans un appareil selon la revendication
6, le réseau de capteurs (20) comprenant des capteurs de pression du type à fibres
optiques décalés radialement par rapport à un axe du réseau (20) et disposés sensiblement
dans le même plan orthogonal audit axe, des moyens (40) pour transmettre un signal
de commande aux capteurs et des moyens (42) pour transmettre des signaux de données
desdits capteurs, caractérisé en ce que trois au moins desdits capteurs (30, 32, 34,
36) du type à fibres optiques sont disposés de façon que leurs axes s'étendent sensiblement
le long de différents rayons du réseau, afin que lesdits capteurs produisent des signaux
de données comprenant des composantes à courant continu représentatives de l'amplitude
de la pression hydrostatique et des composantes à courant alternatif représentatives
des transitoires dues aux ondes sismiques.